For a beginner level player – Don’t focus on openings. Focus on improving your tactical skills. It will take you a long way up until at least 1600 levels.
Don’t give up. You will have lots of losses and tons of bad tournaments. You learn from your losses more than your wins. Focus on the game rather than the outcome.
Don’t be afraid of the opponent’s rating. You might be scared of a higher rated player and try to play solidly, that will only help your opponent beat you, remember that your opponent is also afraid of losing to you so just keep that in mind.